1. Field of the Invention
This invention generally pertains to traveling wave LiNbO3 intensity modulators and more specifically to a device in which the reduction of electrode loss due to leaky mode coupling is achieved in a traveling wave LiNbO3 intensity modulator, without a reduced substrate thickness.
2. Description of the Related Art
Traveling wave LiNbO3 intensity modulators are of great interest for analog radio frequency (RF), microwave link and digital and analog optical communication applications. Of particular interest is the drive voltage of the modulator as this quantity determines link gain, sensor sensitivity, and drive power requirements for high-speed (40 Gbits) digital links. In velocity matched, traveling wave devices drive voltage is determined by, first, the low frequency voltage-length product, secondly, by velocity and impedance match, and lastly, by electrical losses in the traveling wave electrode structure.
In general these devices are Mach-Zehnder interferometers operated with a push-pull electrode structure, so that fields of opposite polarity operate on each arm of the waveguide. These fields serve to change the index of the electro-optic LiNbO3, which in turn alters the phase of the light traveling in each waveguide, and thus allows operation of the interferometer.
For Z-cut LiNbO3 devices, the electrode structure is typically coplanar waveguide (CPW). CPW is known to be an intrinsically leaky structure in general, and in optical modulator devices. SEE, Rutledge et al., INFRARED AND MILLIMETER WAVES, Vol.10, MILLIMETER COMPONENTS AND TECHNIQUES, Part II, K. J. Burton, Ed., Academic Press, Inc., New York, 1983; and Gopalakrishnan et al., ELECTRICAL LOSS MECHANISMS IN TRAVELING WAVE LiNbO3 OPTICAL MODULATORS, Electron. Lett., Vol. 28, No. 2, pp. 207-208, 1992 Coupling can occur both between the guided mode and radiation modes in the substrate, and between the guided mode and slab, or substrate, modes in the substrate. Once power is coupled out of the guided mode, it is lost and cannot contribute to optical modulation, thus resulting in an increase in measured drive voltage.
One approach used previously to control coupling of the guided mode to substrate modes was to employ very thin substrates, xcx9c0.25 mm or less. SEE, U.S. Pat. No. 5,416,859, Burns et al., issued May 16, 1995. This has the effect of changing the mode dispersion of the slab, so that the undesirable mode coupling only occurred at higher frequencies, out of the range of interest. This approach was effective, but thin substrates were very fragile and hard to work with, resulting in a low yield of surviving devices. This is particularly so as device length increased to reduce drive voltage.
These leaky mode effects increase with frequency, as the calculated loss coefficient for the guided mode is proportional to the cube of the frequency, f3, for coupling to radiation modes, and to the square of the frequency, f2, for coupling to substrate modes. In both cases, the loss coefficient is proportional to the square of the overall waveguide width, WtotH=SH+2WH, as defined in FIG. 1b. This implies that the widest part of the waveguide horn is responsible for the largest part of the leaky mode loss, and that this extra propagation loss can be minimized by keeping the horn structure small. However, for packaged devices the end of the horns must be sufficiently large that they can be contacted to a microwave connector either directly or with wire bonding or some other connection method.
An object of this invention is to control leaky mode loss without having a thin substrate.
Another object of this invention is to make a device having a large enough horn size that connection can be made to a electronic transmission cable either directly or with wire bonds.
Another object of this invention is to make a device that will operate to 40 GHz and beyond.
These and other objectives are accomplished by a low loss coplanar waveguide horn with low drive voltage LiNbO3 modulators wherein the electrical transmission of the traveling wave electrode structure, and of the input and output coupling structures, sometimes called the xe2x80x9chornsxe2x80x9d, which transition the active electrode structure with microwave connectors, is maximized by an appropriate design of the horn structure. The conflicting requirements of the leaky mode loss and maintaining sufficient horn size to allow microwave connection, can be reconciled by an adjustment of the ground plane width, which also effects the magnitude of coupling of the guided mode to substrate and radiation modes. Control of both the maximum horn size and the width of the ground planes in particular modulator electrode structures can provide operation to 40 GHz in LiNbO3 devices with substrate thicknesses of xcx9cxe2x89xa61.0 mm, without excess leaky mode loss. The horn structures are xe2x89xa63 mm long and the active waveguides are xcx9c4-5 cm long. They are made of Z-cut LiNbO3, with electroplated gold CPW electrodes of 10-30 xcexcm thickness.